1. Field of the Invention
This invention relates, in general, to the method of fabricating a phase shift mask, and, more particularly, to the method of fabricating an improved single layer phase shift mask with an opaque layer.
2. Description of the Related Art
A photo mask is a tool for transferring patterns in the photolithography process. The main part of the mask is a flat and transparent plate composed of such materials as glass or quartz. A chrome dioxide layer, which includes the transferred patterns for defining semiconductor devices, is formed on the flat and transparent plate. The thickness of the chrome dioxide layer is in the hundreds of .ANG..
Currently, a phase shift mask is predominantly used in the photolithography process. It can provide better resolution for transferred patterns projected on a wafer by a projection machine. The phase shift mask has one shift layer on the conventional mask. The function of the shift layer is to generate positive and negative phase light interference when administering an exposure step in the photolithography process. Therefore, the phase shift mask can provide better resolution for transferred patterns projected on the wafer by the projection machine.
Typically, the phase shift mask has two kind of structures. One structure of the phase shift mask is a single layer phase shift mask. The other structure is a double layer phase shift mask. A process flow, showing the formation of a conventional single layer phase shift mask, is illustrated in FIGS. 1A and 1B. A process flow, showing the formation of a conventional double layer phase shift mask, is illustrated in FIG. 2A to FIG. 2E. As illustrated in FIG. 1A, a phase shift layer 102, such as chrome oxide, is formed over a transparent substrate 100, such as glass or quartz. Then, as shown in FIG. 1B, the pattern of the phase shift layer 102 is formed by exposing, developing and etching. The phase shift layer 102 including chrome oxide is a semi-transparent layer. Patterns of reticule marks or stepper marks on the mask are also semi-transparent. Although the single phase shift layer can provide better resolution for transferred patterns projected on the wafer by a projection machine, the semi-transparent reticle mark or stepper mark on the mask can easily cause registration deviation because of the high transmittance of chrome oxide. Therefore, the reticle mark or stepper mark easily causes a problem of misalignment.
Next, in FIG. 2A, a phase shift layer 202, such as MoSiON, is formed over a transparent substrate 200, such as glass or quartz. An opaque layer 204, such as chrome, is deposited on the phase shift layer. As shown in FIG. 2B, the pattern of the opaque layer 204 is defined by exposing, developing and etching. Next, in FIG. 2C, the pattern of the phase shift layer 202 is formed by a dry etching method. Then a photoresist layer 206 is painted and a pattern of the photoresist layer 206 is formed, as shown in FIG. 2D. Finally, the last pattern of the opaque layer 204 is formed by exposing, developing and etching, as shown in FIG. 2E. The conventional double layer phase shift mask is, therefore, completed.
The double layer phase shift mask is composed of a phase shift layer, such as MoSiON, and an opaque layer, such as chrome, for covering the patterns of the reticle mark or the stepper mark. Therefore, when using the double layer phase shift mask in the photolithography process, the double layer phase shift mask can provide better resolution for transferred patterns projected on the wafer. Moreover, it can avoid the registration deviation and effectively reduce the problem of misalignment of the reticle mark or the stepper mark. The method of fabricating a double layer phase shift mask includes two photolithography steps and one dry etching step, requiring frequent changes of operation machines. Therefore, the method of fabricating a double layer phase shift mask is complex and burdensome, increasing the process time and the process cost greatly.